1. Field of the Invention
The present invention relates to a reflection liquid crystal display and, more particularly, to a reflection liquid crystal display capable of displaying pictures having improved color purity.
2. Description of the Related Art
There is a known color liquid crystal display of an electric field-controlled birefringence system (hereinafter referred to as xe2x80x9cECB systemxe2x80x9d).
In an STN liquid crystal display, a liquid crystal layer has a birefringent effect to split a light beam. Therefore, when white light linearly polarized by a back polarizing plate travels through the liquid crystal layer, there occurs wavelength dispersion causing the combination of elliptically polarized light beams having major axes of different directions depending on wavelength. Therefore, different transmittance curves for light beams of different wavelengths, such as red, green and blue light beams, are obtained when voltage applied across the liquid crystal layer is varied gradually to change xcex94nd which is a product of xcex94n and d (xcex94n is the index anisotropy of the liquid crystal layer and d is the thickness of the liquid crystal layer).
When the product xcex94nd in a state where any voltage is not applied across the liquid crystal layer is not greater than a predetermined value and a birefringent layer having xcex94nd substantially equal to that of the liquid crystal layer is interposed between a liquid crystal cell and a front polarizing plate, the birefringent effect of the liquid crystal layer in a state where any voltage is not applied across the liquid crystal layer can substantially completely be cancelled out and light of characteristics nearly equal to those of the incident white light can be emitted. Therefore, pictures can be displayed in different colors, such as red, green and blue, by continuously varying the xcex94nd of the liquid crystal layer by controlling the voltage applied across the liquid crystal layer. Thus, color change can be achieved by voltage control.
The ECB system does not employ any color filter having disadvantages, such as high manufacturing cost and low transmittance. Therefore, the liquid crystal display of the ECB system displays pictures on a bright screen, does not consume much power and can be manufactured at a low manufacturing cost. The ECB system can be applied to both transmission liquid crystal displays and reflection liquid crystal displays.
Referring to FIG. 3, a reflection liquid crystal display 51 of the ECB system has a liquid crystal layer 52, an upper glass substrate 53 and a lower glass substrate 54. An upper transparent electrode layer 55 and an upper alignment layer 56 are formed in that order on the inner surface of the upper glass substrate 53. A lower transparent electrode layer 57 and a lower alignment layer 58 are formed in that order on the inner surface of the lower glass substrate 54.
The liquid crystal layer 52 is sandwiched between the upper alignment layer 56 and the lower alignment layer 58. A phase plate 59 capable of functioning as a birefringent layer, and an upper polarizing plate 60 are placed in that order on the outer surface of the upper glass substrate 53. A lower polarizing plate 61 and a reflecting plate 62 are placed in that order on the outer surface of the lower glass substrate 54. The reflecting plate 62 is formed by coating an irregular surface of a polyester film 65 with a metal reflecting film 63 of aluminum or silver by evaporation or the like. The metal reflecting film 63 has an irregular surface 64. The reflecting plate 62 is placed on the lower polarizing plate 61 with the irregular surface 64 of the metal reflecting film 63 in contact with the lower polarizing plate 61.
The reflection liquid crystal display displays pictures by using only sunlight or illuminating light and does not use any backlight. Although the reflection liquid crystal display has the advantage of operating at low power consumption, the lightness of pictures displayed by the reflection liquid crystal display is somewhat lower than that of pictures displayed by the transmission liquid crystal display provided with a backlight.
The liquid crystal display of the ECB system displays colors only by controlling the voltage applied across the liquid crystal layer without using any color filters. However, the liquid crystal display of the ECB system has difficulty in clearly displaying red and green in a satisfactory color purity.
Accordingly, it is an object of the present invention to provide a reflection liquid crystal display of the ECB system capable of displaying pictures in improved lightness and of displaying particularly red and green in improved color purity.
According to one aspect of the present invention, a reflection liquid crystal display comprises a first transparent substrate, a second transparent substrate disposed opposite to the first transparent substrate, and a liquid crystal layer sandwiched between the first and the second transparent substrate; wherein a transparent electrode layer and an alignment layer are formed in that order on the inner surface of the first transparent substrate, a reflecting polarizing film formed by laminating a transparent scattering layer and a light absorbing layer is placed on the outer surface of the first transparent substrate, a transparent electrode layer and an alignment layer are formed in that order on the inner surface of the second transparent substrate, and a phase plate and a polarizing plate are placed in that order on the outer surface of the second transparent substrate.
The reflection liquid crystal display in accordance with the present invention is provided with the reflection polarizing film instead of the polarizing plate and the reflecting plate to improve the color purity of red and green.
Preferably, the liquid crystal layer of the reflection liquid crystal display has a helical structure twisted through an angle in the range of 240xc2x0 to 260xc2x0, a value xcex94n1d1 which is a product of xcex94n1 and d1, where xcex94n1 is the index anisotropy of the phase plate and d1 is the thickness of the phase plate, is in the range of 1000 to 2000 nm, a value xcex94nd which is a product of xcex94n and d, where xcex94n is the index anisotropy of the liquid crystal and d is the thickness of the liquid crystal layer, is in the range of 800 to 1800 nm, the absorption axis of the polarizing plate is inclined to the delay axis of the phase plate at an angle in the range of xe2x88x9240xc2x0 to xe2x88x9260xc2x0 in a counterclockwise direction as viewed from the side of incident light, the delay axis of the phase plate is inclined to the alignment direction of the second alignment layer on the second transparent substrate at an angle in the range of xe2x88x9265xc2x0 to xe2x88x9285xc2x0 in a counterclockwise direction as viewed from the side of incident light, the absorption axis of the reflecting polarizing film is inclined to the alignment direction of the first alignment layer of the first transparent substrate at an angle in the range of +35xc2x0 to +55xc2x0 in a counterclockwise direction as viewed from the side of incident light.
Only a certain measure of improvement can be expected when the reflection liquid crystal display of the present invention is provided with only the reflecting polarizing film. Further effect can be expected when the polarizing plate, the phase plate and a rubbing axis are arranged in an optimum axis arrangement. The inventors of the present invention found through studies of axis arrangement that the foregoing axis arrangement further enhances the lightness color clearness of pictures displayed on the reflection liquid crystal display.